1. Field of the Invention
This invention relates to a process for surface-coating base metals and more particularly, it is concerned with a process for surface-coating steels or cemented carbides for tools or parts with hard materials or chemically stable materials by the plasma CVD method.
2. Description of the Prior Art
Up to the present time, tools or parts have been coated with titanium carbide or titanium nitride for the purpose of improving the wear resistance, heat resistance and corrosion resistance thereof. In particular, cutting tools consisting of cemented carbides, as substrates, coated with hard coating layers have lately been generalized. Typical of the coating methods are CVD methods and PVD methods and in cutting tools, in particular, the CVD method has played a most important part. Since the CVD method employs a higher coating temperature, e.g. 1000.degree. C., in general, a substrate or base material of steel is annealed and thus it is required to effect quenching and tempering after coating. Accordingly, there take place cracking, flaking-off and change of the dimension in a coating layer and it is difficult to produce an article having a good precision in stable manner. In the CVD method, furthermore, a decarburization phenomenon sometimes occurs in a substrate during the reactions and in such a case, it is very difficult to form a coating layer without deteriorating the substrate.
In the PVD method, on the other hand, the above described problems have been solved since formation of a coating layer is possible even at a lower temperature, e.g. 600.degree. C. or lower, so annealing or decarburization of a substrate does not occur. Moreover, a sufficient bonding strength of a substrate and coating layer has been obtained by various improvements even if the coating is carried out at a lower temperature and consequently, the PVD method has held an important position in processes for the production of coated steels. However, the PVD method is inferior to the CVD method in respect of that in the former method using a high vacuum, the installation cost is higher, the resulting coating has a directional property, a revolving mechanism is required because of the lower deposition efficiency and the producibility is remarkably lower, thus resulting in a higher processing cost. In addition, coating of a part having a complicated profile is difficult because of the lower uniformity of the coating layer thickness.
In the case of coating cemented carbides by the CVD method, the strength of the cemented carbide substrates is lowered after the coating, so that the edges tend to break under severer cutting conditions, for example, in heavy cuttings and the lives are not so lengthened. Therefore, the CVD method is not suitable as a coating method of a tool for heavy cutting. On the other hand, the PVD method has advantages that the strength of a substrate is not lowered, the edge is hardly broken even in heavy cutting and the coating layer can exhibit the intrinsic effect of preventing a workpiece from adhesion to the surface of a tool, thus lengthening the tool life. However, the PVD method has also disadvantages that the installation cost is higher, a special mechanism for revolving a substrate is required because of the lower uniformity of the coating layer thickness and the reproducibility is remarkably lower than in the CVD method, thus resulting in a greater processing cost.